Process for chemical vapor deposition of a liquid raw material

ABSTRACT

A chemical vapor deposition process is disclosed comprising the steps of forming liquid droplets of a liquid raw material; injecting the liquid droplets through a plate member opening placed opposite to the surface of the substrate to vaporize the liquid droplets; and supplying a reaction gas that reacts with the vaporized raw material; and depositing a thin film on the substrate.

This application is a division of application Ser. No. 07/995,040 filedDec. 22, 1992, now U.S. Pat. No. 5,421,895.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an apparatus for vaporizing a liquidraw material, and more particularly to the apparatus applicablepreferably for depositing a thin film by chemical vapor deposition.

2. Related Background Art

In a step of forming a thin film in the process for producing anintegrated circuit with semiconductors, it is necessary to form adesired thin film on an etched wafer surface of uneven surface levelwhile attaining a good step coverage. In a process for producing anintegrated circuit of submicron level, whose minimum processingdimension is not more than 1 μm, or of further deep submicron level, itis necessary to deposit a thin film selectively in contact holes orthrough holes. It is a step of forming a wiring metal film that requiresthe keenest selective growth in the process for producing an integratedcircuit.

A thin film has been so far formed conventionally by sputtering.Sputtering is a process which deposits atoms sputtered from a targetsubstance onto a substrate. Thus not only is its step coverage poor, butit is also impossible to attain selective growth. On the other hand,chemical vapor deposition, which vaporizes a raw material, transportsthe vaporized raw material into a reaction space and deposits a desiredthin film through chemical reactions of the raw material molecules onthe substrate surface, provides distinguished step coverage and canprovide selective growth. For example, an Al thin film growth processdisclosed in Applied Physics Letters, Vol. 57, No. 12 (1990), Page 1221,can deposit monocrystalline Al selectively in very fine through-holesand also can change the growth mode from a selected mode to anon-selected mode to completely flatten through-holes.

In the chemical vapor deposition process (CVD), it is necessary tovaporize a raw material and transport the vaporized raw material into areaction space. If the raw material exists in a gaseous state in theCVD, its transportation into the reaction space can be carried out withease. For example, SiH₄ is a gas at room temperature and thus can befilled in a high pressure gas cylinder. Therefore, it can be transportedinto the reaction space at a desired rate by providing the high pressuregas cylinder with a pressure control valve and a flow meter. However,when a raw material is a liquid at room temperature, its transportationprocedure is different from that for SiH₄. For example,dimethylaluminum, which is a liquid at room temperature, is used as araw material in the Al, thin film growth. Such procedure is disclosed indetail in U.S. patent application Ser. No. 578,672, now U.S. Pat. No.5,179,042 titled "Process for forming deposited film by use of alkylaluminium hydride", filed Sep. 7, 1990 by the present inventors; U.S.patent application Ser. No. 587,045, now U.S. Pat. No. 5,180,687 titled"Deposited film formation method utilizing selective deposition by useof alkyl aluminum hydride", filed Sep. 24, 1990 by the presentinventors, and U.S. patent application Ser. No. 586,877 titled "Gasfeeding device and deposition film forming apparatus employing the same"filed Sep. 24, 1990, now U.S. Pat. No. 5,476,547 by the presentinventors.

FIG. 3 shows a procedure for transporting a liquid raw material which iswidely utilized, where a liquid raw material 2 is stored in a vessel 1provided with an inlet pipe 103 and an outlet pipe 104, and the inletpipe 103 is dipped into the liquid raw material 2. When a flowrate-controlled gas 105 is introduced from the inlet pipe 103, the gas106 is discharged from the outlet pipe 104 through the liquid rawmaterial 2. Since the gas 105 is passed through the liquid raw material2, the gas 106 can be saturated with the vapor of the liquid rawmaterial 2 and led to a reaction chamber of an apparatus for chemicalvapor deposition. The amount of the transported raw material accordingto the foregoing procedure can be represented approximately by thefollowing equation: ##EQU1## wherein Q: Amount of transported rawmaterial

Q_(in) : Amount of gas introduced from the inlet pipe

P_(V) : Saturated vapor pressure of raw material gas

P_(B) : Pressure inside the vessel

When the liquid raw material has a low saturation vapor pressure, theamount of transported raw material is small, as is seen from theequation (1), and in the worst case, the rate of deposition is governedby the amount of the transported raw material. When the pressure P_(B)inside the vessel is constant, it is possible to introduce dilution gasinto the reaction chamber to lower a proportion of the raw materialpartial pressure in the reaction chamber, but it is impossible toelevate the proportion. In order to enhance the transportationefficiency of the raw material, it is possible to elevate thetemperature of the vessel to increase the saturated vapor pressure ofthe raw material, but the entire vessel and pipes must be heated. Whenthe inlet gas flow rate is increased in the procedure as shown in FIG.3, the vaporization rate of the liquid raw material cannot meet theinlet gas flow rate, and thus a sufficient amount of the raw materialcannot be transported.

In the procedure as shown in FIG. 3, the amount of the gas 105introduced into the inlet pipe 103 can be exactly determined, but theamount of the raw material discharged from the outlet pipe 106 can beonly calculated from the equation (1), and its actual amount cannot bedetected, that is, the exact amount of transported raw material can notbe determined.

In the above-mentioned Al CVD using dimethylaluminum hydride (DMAH),deposition of high quality Al film can be conducted, but the saturatedvapor pressure of DMAH at room temperature is as low as 2 Torr and therethus remains the problem that a large amount of DMAH can not beefficiently transported and no higher deposition rate can be attained.

As mentioned above, it is necessary in chemical vapor deposition using aliquid raw material in the technical field of semiconductor integratedcircuits of higher speed and higher degree of integration due to therecent trends for miniaturization to transport a large amount of rawmaterial stably and exactly. Accordingly there still remain unsolvedproblems for the mass production.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a means of efficientlyvaporizing even a raw material having a low saturated vapor pressure atroom temperature efficiently and transporting a large amount of thevaporized raw material exactly and also to provide an apparatus capableof transporting a large amount of raw material stably and exactly in CVDusing a liquid raw material.

According to one aspect of the present invention, there is provided anapparatus for vaporizing a liquid raw material, which comprises a nozzlewith an open tip end for ejecting a liquid raw material into a heatedgas atmosphere as liquid droplets and a heated plate with a smallopening disposed in front of the nozzle, a space opposite to thatoccupied by the nozzle with respect to the plate being evacuated.

According to another aspect of the present invention, there is providedan apparatus for forming a thin film, which comprises theabove-mentioned apparatus for vaporizing a liquid raw material, where alarge number of substrates can be treated stably.

In the transportation of a large amount of a liquid raw material, it isnecessary to vaporize a liquid raw material efficiently in a hightemperature circumstance for elevating the saturated vapor pressure. Ina bubbling system as shown in FIG. 3, a sufficiently large amount of aliquid raw material is not vaporized even by heating the vessel 1appropriately, and it is thus quite difficult to transport a largeamount of the raw material.

In the present invention, on the other hand, a nozzle for ejecting aliquid raw material in a liquid droplet state into a gas heated to atemperature lower than the decomposition temperature of the liquid rawmaterial is provided, where the liquid raw material can be ejected asliquid droplets having a diameter of 10 to 50 μm into the heatedatmosphere gas to be vaporized efficiently.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of the main part of anapparatus for vaporizing a liquid raw material according to the presentinvention.

FIG. 2 is a schematic cross-sectional view of an apparatus fordepositing a thin film, to which the present apparatus for vaporizing aliquid raw material is applied.

FIG. 3 is a cross-sectional view of a conventional liquid raw materialbubbler.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferable embodiments of the present invention will be explained indetail below, referring to the drawings.

According to the present invention, a liquid raw material is transportedin a liquid state and ejected into a heated gas in a reaction chamber,thereby completely incorporating a vapor of the liquid raw materialhaving its saturated vapor pressure into the heated gas.

FIG. 1 schematically shows the region for ejecting a liquid raw materialand the region for vaporizing the ejected liquid raw material, where anozzle 4 has a tip end with a reduced cross sectional area, throughwhich a liquid raw material 2 is transported at a controlled flow rate,and a piezoelectric vibrator 8 gives vibrations to the tip end of nozzle4. When the liquid raw material 2 is led to the nozzle tip end in thisstructure while vibrating to the nozzle tip end, the liquid raw material2 is ejected in liquid droplets having a diameter of about 10 μm toabout 50 μm from the nozzle tip end due to the nozzle vibrations.

A plate 30 having a small opening 40 is provided in front of and in thedirection of the nozzle tip end. A heater (not shown in the drawing) isembedded in the plate 30 to heat the plate 30. An inert gas or H₂ gas ortheir mixed gas 31 flows into the opening 40 from the nozzlesurrounding.

At that time, space A is under a pressure of 500 to 760 Torr, whereasspace B, which is a reaction chamber, is under a pressure for depositinga thin film, for example, 0.1 to 10 Torr. A pressure gradient developsin the opening 40 from the space A towards the space B. The space B isevacuated by an evacuating means (not shown in the drawing) and keptunder the predetermined pressure. The pressures of spaces A and B arenot limited to the above-mentioned values. In principle, as long as thepressure of space A is higher than that of space B, any values areavailable. That is, the space A can be in a pressurized state of 1 to 2kg/cm². When a liquid droplet 50 is ejected from the nozzle 4 into thespace A, small opening 40 and space B, the liquid droplet 50 isvaporized in the small opening 40 and introduced as a gas into the spaceB.

When the small opening 40 is at room temperature, the liquid droplet 50will not be vaporized, but the plate 30 is heated and the heated gas 31is introduced, and thus the liquid droplet ejected from the nozzle canbe vaporized in the small opening. Different from the conventionalprocedure of heating the entire vessel and the entire pipes to increasethe amount of transported liquid raw material, as shown in FIG. 3, thepresent vaporizing structure, as shown in FIG. 1, can exactly determinethe amount of liquid raw material as transported and can increase theamount of liquid raw material up to the corresponding saturated vaporpressure at an elevated temperature.

FIG. 2 schematically shows one embodiment of an apparatus for chemicalvapor deposition, provided with the apparatus for vaporizing a liquidraw material, shown in FIG. 1, where a vessel 1 containing a liquid rawmaterial 2 is pressurized with a pressurizing gas from a gas inlet 11through a valve 10 and the applied pressure is adjusted with a pressuregage 9. As the pressurizing gas, any gas may be used so long as it isunreactable with the liquid raw material 2. For example, when the liquidraw material is an organometallic compound, an inert gas such as Ar, N₂,He, Ne, etc. can be used. When trimethylaluminum, dimethylaluminumhydride, trimethylgallium or the like is used as a liquid raw material,these organometals decompose in the presence of a hydrogen gas even atroom temperature, and thus Ar, N₂, He, Ne or the like is preferable.From the viewpoint of low cost and high purity, Ar or N₂ is morepreferable.

When the vessel 1 is pressurized by the pressurizing gas from the gasinlet 11, the liquid raw material 2 is led to a liquid flow rate meter 3on the basis of the principle of siphon. A commercially available liquidmass flow controller can be used as the liquid flow rate meter. Any flowrate meter can be used, so long as it can exactly determine the flowrate of the liquid raw material. After the passage through the liquidflow rate meter 3, the liquid raw material is led to nozzles 4 to 7.

In FIG. 2, only four nozzles are shown, but less or more than fournozzles can be used. In FIG. 2, the nozzles are shown as if they aredisposed in a straight line, but disposition of nozzles in a concentricstate in a plane is desirable. Each of the nozzles 4 to 7 is providedwith a piezoelectric vibrator 8, and liquid droplets are ejected fromthe individual nozzle tip ends.

In the embodiments shown in FIGS. 1 and 2, the liquid raw material isejected into a liquid droplet state from the individual nozzle tip endsby vibrations of the piezoelectric vibrators. Any other means than thepiezoelectric vibrators can be used, if they can eject liquid dropletshaving a diameter of 10 to 100 μm. Space 20 is set to approximately theatmospheric pressure, whereas space 21 is set to an appropriate pressureof 0.1 to 10 Torr for formation of a desired deposited film. Pressuredifference between the space 20 and the space 21 is created by smallopenings 40 provided through a baffle plate 30. The small openings 40 ofthe baffle plate 30 are each disposed in front of the nozzle tip ends.The number of the small openings 40 can be equal to or more than thenumber of the nozzles. Disposition of the small openings to theindividual nozzle tip ends is as in FIG. 1. Near each of the smallopenings and each of the nozzle tip ends, gas ejection outlets 14 areprovided.

FIG. 2 shows as if only one gas ejection outlet 14 is provided for onlythe tip end of nozzle 7, but actually gas ejection outlets 14 areprovided for the individual nozzles 4 to 7. The gas from the gasejection outlets 14 is heated in advance by heaters 41. The temperatureof nozzles 4 to 7 is set to a temperature at which the liquid rawmaterial may not be decomposed. Numeral 12 is a gas inlet and 13 is agas flow rate meter. A gas may be introduced into the space 20 through apipe 17 via a flow rate meter 16 from a piping inlet. As explainedreferring to FIG. 1, the baffle plate 30 is heated by a heater (notshown in the drawing), and the liquid droplets ejected from the nozzles4 to 7 are vaporized in the small openings 40 provided through thebaffle plate 30.

The baffle plate 30 may be of a metal or of an insulating material. Incase of an organometal as a liquid raw material, decomposition reactionof the organometal is promoted on a heated metal, and thus an insulatingmaterial such as quartz or BN (boron nitride) is preferable.Furthermore, from the viewpoint of excellent processability and easyembedment of a heater for heating, BN (boron nitride) is more preferablefor the baffle plate 30. The baffle plate 30 needs to have a thicknesssuch that the liquid droplets ejected from the nozzles 4 to 7 can befully vaporized in the heated spaces of the individual small openings40. In the embodiment shown in FIG. 2, the thickness is about 5 mm toabout 3 cm. Basically the size of the individual small openings 40 needto be larger than the diameter of the liquid droplet, and is about 0.5to about 2 mm in diameter. In the reaction chamber 21, a substrate 25 isplaced on a substrate support 24. In the embodiment shown in FIG. 2, thesubstrate 25 is heated by a heater from the back side of the substrate25. The substrate 25 can be heated by high frequency heating or by lampheating. The reaction chamber 21 is connected to an evacuating valve 26and to evacuating means 27. To adjust the reaction chamber 21 to adesired pressure, the degree of opening of the valve 26 is controlledwhile observing the pressure gage 22.

The apparatus for vaporizing a liquid raw material shown in FIG. 1 asone embodiment of the present invention and the apparatus for depositinga thin film shown in FIG. 2 as one embodiment of the present inventioncan be utilized in all the apparatuses for chemical vapor depositionusing a liquid raw material. For example, organometallic raw materialssuch as trimethylaluminum, dimethylaluminum hydride, triisobutylaluminumhydride, trimethylgallium, etc. as a liquid raw material ortetraethylsilane can be used as a raw material. Particularly, thepresent invention is effective for dimethylaluminum hydride (vaporpressure at room temperature: about 1 to about 2 Torr) andtriisobutylaluminum (vapor pressure at room temperature: about 0.1 Torr)having a low vapor pressure at room temperature. Furthermore, aplurality of gases can be used as raw material gas. For example, when aGaAs thin film is formed, trimethylgallium is ejected from the nozzles,and AsH₃ is introduced into the reaction chamber 21. In the apparatusfor depositing a thin film as shown in FIG. 2, only one kind of theliquid raw material is given, but a GaAlAs liquid crystal thin film canbe also deposited with trimethylgallium from one vessel andtrimethylaluminium from another vessel.

An embodiment of depositing a thin film in the apparatuses as shown inFIGS. 1 and 2 will be described in detail below, where dimethylaluminiumhydride was used as a liquid raw material, an inert gas Ar was used as apressurizing gas from the gas inlet 11, and nozzles 4 to 7 made ofstainless steel were used. Liquid droplets having a diameter of 10 to 50μm were ejected from the nozzles 4 to 7 with the vibrators 8 at afrequency of 1 to 50 kHz. H₂ gas was used as the gas from the gas inlets14 and adjusted to an inlet gas temperature of about 150° C. at theejection outlets 14. The inlet gas temperature was preferably lower thanthe decomposition temperature of dimethylaluminum hydride, 160° C. H₂gas was likewise introduced from the gas inlet 12. A BN plate embeddedwith a heater was used as the baffle plate 30, and the reaction chamber21 was adjusted to a pressure of 0.1 to 3 Torr, whereas the space 20 wasunder a pressure of the atmospheric pressure to 500 Torr. Under theseconditions, the liquid droplets ejected from the nozzles 4 to 7 werevaporized in the small openings 40, and a large amount ofdimethylaluminum hydride could therefore be introduced into the reactionchamber 21 to deposit a thin Al film. When the substrate temperature wasat about 270° C., an Al film of excellent film quality and surfaceflatness could be deposited.

When dimethylaluminum hydride was bubbled at a flow rate of 100 SCCM ormore under a vessel inside pressure of the atmospheric pressure at roomtemperature according to the conventional system shown in FIG. 3, theamount of the transported raw material was not proportional to thebubbling flow rate due to the high viscosity and was thereforesaturated. On the other hand, in the apparatuses of the presentinvention, as shown in FIGS. 1 and 2, the dimethylaluminum hydride couldbe transported proportionally to the transported amount of the liquid,and a high speed deposition of 1 μm/min. could be attained on a 4-inchwafer.

As explained above, a liquid raw material having even a low vaporpressure can be ejected into a high temperature atmosphere as liquiddroplets and a large amount of the raw material can therefore betransported in the present invention.

What is claimed is:
 1. A chemical vapor deposition process comprisingthe steps of:arranging a substrate in a first chamber that is separatedfrom an adjacent second chamber by a baffle plate having a plurality ofopenings provided therethrough; making the pressure of the secondchamber higher than that of the first chamber; supplying a liquid rawmaterial to plurality of nozzles provided in the second chamber througha flow rate meter; heating the substrate; injecting liquid droplets ofthe raw material from the plurality of nozzles toward the plurality ofopenings; heating the baffle plate; and effecting deposition of a thinfilm on the substrate.
 2. The process according to claim 1, wherein theraw material comprises alkyaluminium hydride.
 3. The process accordingto claim 1, wherein the raw material comprises dimethylaluminiumhydride.
 4. The process according to claim 1, wherein the raw materialcomprises any one of trimethylaluminium, trimethylgallium andtetraethylsilane.
 5. The process according to claim 1, furthercomprising a step of introducing heated hydrogen gas into the secondchamber.
 6. The process according to claim 2, further comprising a stepof introducing heated hydrogen gas into the second chamber.
 7. Theprocess according to claim 3, further comprising a step of introducingheated hydrogen gas into the second chamber.
 8. A chemical vapordeposition process comprising the steps of:arranging a substrate in afirst chamber that is separated form an adjacent second chamber by aplate having a plurality of openings provided therethrough; making thepressure of the second chamber higher than that of the first chamber;supplying a liquid raw material to a plurality of nozzles provided inthe second chamber through a flow rate meter; heating the substrate;injecting liquid droplets of the raw material from the plurality ofnozzles toward the plurality of openings; heating the plate; andeffecting deposition of a thin film on the substrate.
 9. The processaccording to claim 8, wherein the raw material comprises alkyaluminiumhydride.
 10. The process according to claim 8, wherein the raw materialcomprises dimethylaluminium hydride.
 11. The process according to claim8, wherein the raw material comprises any one of trimethylaluminium,trimethylgallium, and tetraethylsilane.
 12. The process according toclaim 8, further comprising a step of introducing heated hydrogen gasinto the second chamber.
 13. The process according to claim 9, furthercomprising a step of introducing heated hydrogen gas into the secondchamber.
 14. The process according to claim 10, further comprising astep of introducing heated hydrogen gas into the second chamber.